The many benefits and methods of mounting a Very High Frequency Ultrasound (“VHFU”) probe on an arcuate guide track have been discussed in prior patents and published patent applications. Ultrasound imaging systems are described in each of the following patent applications, all of which are incorporated by reference:
1. U.S. Pat. No. 8,317,709 entitled “Alignment and Imaging of an Eye with an Ultrasonic Scanner” issued Nov. 27, 2012.
2. U.S. patent application Ser. No. 12/347,674, entitled “Innovative Components for an Ultrasonic Arc Scanning Apparatus” filed Dec. 31, 2008.
3. U.S. patent application Ser. No. 12/754,444 entitled “Method of Positioning a Patient for Medical Procedures” filed Apr. 5, 2010.
4. U.S. patent application Ser. No. 12/418,392 entitled “Procedures for an Ultrasonic Arc Scanning Apparatus” filed Apr. 3, 2009.
Ultrasonic imaging has been used in corneal procedures such as LASIK to make accurate and precise images and maps of cornea thickness which includes epithelial thickness, Bowman's layer and images of LASIK flaps. These images have an A-scan resolution of about 35 microns and have been shown to attain repeatability of better than about 1 micron. This repeatability is discussed in “Repeatability of Layered Corneal Pachymetry with the Artemis Very High Frequency Digital Ultrasound Arc-Scanner” by D. Z. Reinstein, T. J. Archer, M. Gobbe, R. H. Silverman and D. J. Coleman, MD in the Journal of Refractive Surgery November 2009.
A key contribution to the accurate and precise cornea layer measurement achieved by ultrasonic imaging systems is attributed to the arcuate movement of the ultrasound probe around the cornea surface such that the probe remains substantially perpendicular to the cornea surface at all times during the scan. Maintaining the ultrasound probe aligned substantially perpendicular to the cornea surface during the scan assures maximum possible reflectance of the ultrasound beam from the cornea and provides superior signal to noise ratios of echoes of not only the anterior and posterior surfaces of the cornea, but all intermediate biologic interfaces, most notably of Bowman's interface which separates the epithelium from the stroma.
Given the high diagnostic value of the epithelium thickness map this ultrasound imaging technology provides, other imaging technologies are now being adapted to provide similar capabilities of epithelium thickness mapping. A promising example is Optical Coherence Tomography (“OCT”) systems which use infrared light. OCT systems have the potential to produce higher image resolution based on the relatively small wavelength of the infrared light they use compared to the long wavelength used by VHFU platforms. OCT systems have been developed for both general anterior segment imaging and retinal scanning OCT systems have been demonstrated to be a very effective high precision imaging technology, particularly for ocular structures (such as the cornea and the natural lens) that are transparent to the infrared light they use. However, the ability of OCT systems to image into opaque tissues is limited to a depth of about 1 to about 2 mm.
There are several challenges presented in the use of OCT platforms to image an eye. The practically achievable image resolution and precision of an OCT platform is very much a function of the strength of the reflected signal relative to noise. The strength of the reflected signal is driven largely, but not entirely, by the electric field reflectivity profile along the axis of the infrared beam. For generally opaque tissues which present strong reflectors at a number of angles the reflectivity profile is quite strong over a wider range of angles of the incident infrared beam relative to the tissue surface. However, the transparent structures of the eye pose a unique challenge as they are specular and as such provide a very high reflectance for a beam perpendicular to the surface but the rate of reflectance falls off rapidly as the beam strikes the surface at an oblique angle.
Most current OCT systems scan the imaging beam across the image field with the beam at a fixed angle and aligned with the visual axis of the eye. In these OCT systems, as the image beam is scanned further away from the visual axis, the image beam angle relative to the specular surfaces of the cornea layers (as an example) becomes more oblique and reflectance begins to drop quickly. To overcome this problem, some OCT systems employ over-sampling, surface curve fit, eye movement tracking (because of longer sample times from over-sampling) and many other signal and image processing techniques. Although these techniques may generally provide adequate results, these techniques increase the complexity and cost of OCT systems and in some cases increase the amount of time necessary to complete a scan.
The resolution and precision of OCT systems used to image cornea layers is further aggravated by the fact that the impedance differences between the cornea layers is quite small because the cornea is designed to transmit as much light as possible. These challenges, beam alignment to specular surfaces and low impedance difference within the cornea layers, serially combined are very difficult to overcome for commercially useful imaging of the cornea layers with OCT systems.
The fixed angle scanning of most OCT imaging systems develops yet another challenge unique to imaging specular surfaces. All current diagnostic OCT systems are non-contact systems, meaning the OCT beam travels through air before entering the specular surface of the cornea. Because the impedance mismatch between air and the cornea is quite large, as anyone skilled in the art will appreciate, a significant refraction correction must be made particularly as the angle between the incident beam and the cornea surface becomes more oblique. Therefore the thickness measurements of the cornea away from the central axis must also be corrected for refraction errors and poses another source of measurement error.
There remains, therefore, a need for improved systems and methods of OCT imaging that can be used to map the thickness of cornea layers with precision by overcoming the challenges of poor electric field reflectivity profiles, low reflected light amplitude due to lack of beam alignment to specular surfaces of the eye, and refraction corrections that are particularly acute away from the visual axis.